1. Field of the invention
The present invention relates to optical elements of optical products such as cameras, image formation apparatuses in which electrophotographic techniques are used, and relates to a method of producing the optical elements.
2. Description of the Related Art
The grinding method is the most well known method for producing a lens. As an example, Japanese Unexamined Patent Publication No. Sho. 62-203744 discloses the following grinding method.
A grinding bowl having a reciprocal shape of a lens to be ground is prepared in advance. The grinding bowl is pushed against a glass material. Abrasive grains are provided between the grinding bowl and the glass material, so that the grinding bowl can grind the glass material to form a lens. Since super fine abrasive grains are used, there is little grind stress applied to the glass material, and little distortion arising in the inside of the lens. However, this lens producing method can not be conducted unless a shape of the lens to be produced is simple, and thus not all lens shapes can be produced. That is, the shape of the lens is limited, having only a few degrees of freedom.
As higher grades of lens performance become required, techniques for making the lens surface aspherical and more precise become more important. Because it is difficult to produce an aspherical lens using the above grinding method, the aspherical surface is generally formed using a machine controlled by numerical control (hereinafter referred to as "NC"). However, it is difficult to obtain a mirror surface that is suitable for use as a lens after processed using the NC, and a shape error remains in the lens in the order of sub micrometers, and some flaws remain on the processed surface by a working tool.
Ductility mode grinding methods may be used to produce a surface that is suitable for use as a lens, even when grinding fragile materials such as a glass. For example, Japanese Patent Application No. 2-53557 discloses a method of grinding an object to have an aspherical surface. The object to be ground is disposed on a turntable turned by a motor, and a grindstone for grinding the object is disposed in an air spindle turning at a speed of about 10000 rpm. A pulse is detected by a rotary encoder that is directly connected to a rotation axis of the turntable, grinding data is provided to a piezo-actuator on the basis of the detected pulse, and a movable table is continuously moved forward and backward. The air spindle is fed one step for each rotation of the turntable and its position is changed so that the position where the grindstone contacts the object changes. Using this method, it is possible to grind freely the object to asymmetrical and aspherical shapes. Moreover, because it is possible to control a cutting depth of the grindstone against the object in the order of sub-micrometers, fragile materials can be ground to obtain a mirror surface that may be used as a lens.
However, this method is time-consuming, as a significant amount of time is required to grind the aspherical parts of the lens surface. Moreover, a ground surface of the lens after the grinding has some convex or concave flaws in the order of sub micrometers. In the case of requiring to produce a lens with high resolution, there arises a problem where the optical characteristics of the lens are adversely affected by these slight flaws. Thus, a final step is required when processing the optical element such as a lens by the NC.
A polishing method is often used in the final step. For example, the lens surface is further polished with diamond grindstones and a soft pad such as felt, thereby forming a better lens surface.
Three kinds of factors indicating the error of the lens surface are well known.
The first is an undulation (in the order of from several hundreds micrometers to several millimeters).
The second is a surface roughness (in the order of from sub micrometers to several micrometers).
The third is a flaw made by the grinding (in the order of from several dozen micrometers to several hundreds micrometers).
In the above, the undulation of the lens strongly depends on a machine precision. Thus, it is difficult to correct the undulation of the lens surface in the final step, and it is required that the undulation is made quite little in the step of the NC.
On the other hand, the surface roughness may be fully improved in the final step (polishing).
The flaw is in a place between the undulation and the surface roughness. When polishing the flaws to improve the lens surface, the whole lens shape is degraded and the undulation is increased. Furthermore, because these flaws result from contacting tools which hit the lens surface, the flaws can not be reduced as the undulation which can be reduced by the NC.
As mentioned above, the NC may process the object to the complicated lens shape. However, since the NC is conducted by machines, it is impossible to prevent the lens surface from making the flaws. In the worse case, deterioration of the lens characteristics may occur.
Conventionally, a coating method is used to improve the lens. However, its purpose is to prevent a ray of light reflecting on the lens and the coating is conducted along the shape of the lens surface. Therefore, the convex or concave of the lens surface may not be modified.